1. Field of the Invention
The present invention generally relates to electrical connectors, and more particularly relates to high-density electrical connectors used in test and burn-in done on miniaturized electrical components.
This invention is a technology platform that enables the interconnection between high-density electronic devices. It covers the springs and springs arrangements that represent the modular building block of devices to achieve high density interconnection.
2. Background Information
It is standard procedure to test chips or integrated circuits at different production stages to cull out the defective ones. Such tests are often done on printed circuit boards circuits, substrates and similar electronic devices as well. This is done to avoid putting extra time, money and effort into a defective component only to end up having to scrap the component at the end of the production process.
In addition, sockets and connectors are used to mount electronic packages and devices onto substrates, instead of soldering them directly onto those substrates. These connectors and sockets need to have their contact springs located on small center distance, which is usually referred to as pitch, so as to match the pitch of those devices, which are usually referred to as High-Density devices.
3. Prior Art
There exist a number of prior art devices of the type in consideration herein. I will explain in the following paragraphs how each of these prior art devices have addressed some of the problems, but left some other issues unanswered and unresolved. I will explain how my present invention addresses and solves these issues.
1. Force-Deflection Curve and Damage to the Device Contacted by the Springs.
A potential user of my devices complained that his present supplier provides him with test heads, which utilize contact springs similar to the IBM Buckling Beam contact springs. He stated to me that the springs apply such a high force that sometimes they damage the substrate that are tested. So, they start with potentially good substrates, test them and during the test process, the substrates get damaged and become defective. It is almost like testing matchsticks. After each one gets tested, you determine that it “was” a good one.
In U.S. Pat. No. 4,622,514, the IBM “Multiple Mode Buckling Beam Probe Assembly”, the contact springs are basically straight columns of wires, which get compressed by an external force to provide the contact pressure and contact force. There are basically two reasons why any electrical interconnection device needs springs and needs forces applied on to these springs. The first reason is to break through the layers of dirt and oxides that form on the contact surfaces and to reach the pure contact metal. In some cases the required force is only a few grams, in other cases it can be considerably higher. The second reason is to accommodate non-planarity of the contacts on the surface of the device to be contacted. For example, if the device has a non-planarity of say several thousands of an inch between any two adjacent contact pads, then one spring would have this much more deflection than the adjacent spring. The effect of this additional deflection is that the spring with more deflection will exert more force on the contact pad. This higher force could reach a level, where the contact spring would then damage the contact pad and render it defective.
So, in order to prevent any damages to the contact pads, the contact forces need to be kept within tight limits. For this reason, it is customary to select contact springs that are “soft” and have a shallow “force-deflection” curve, also called “spring index”. With a soft spring, you start by applying a small force and gently, gradually increase the force until you reach the desired force level that is enough to break through the layers. Ideally you should stop there. If the device was perfectly flat and the non-planarity was zero, then all the springs should have the same deflection and thus the same contact force. But if one contact pad is lower than the others, then the spring touching this pad would not have been deflected as much as the other springs, consequently the force provided by this spring would be smaller than the force provided by the other springs. So, the force on that low pad would be below the desired level. In order to make this force higher and to make it reach the desired level, the spring working on this low pad needs to be deflected more.
But we cannot selectively deflect each individual spring on its own. All the springs have to be pushed down and deflected equally, by the same amount. So, we push down on all the springs, until the spring acting on the lowest pad reaches a deflection that would provide the desired force. But the springs acting on the higher pads, because of their larger deflection, are now providing a much higher force. The amount of this additional force can be calculated from the force-deflection curve, or the stiffness of the spring.
If the springs used are relatively stiff, this can have a detrimental effect. The excessive down-push on the springs can make the force of the high pads reach a level, where the pads can be damaged. A secondary bad effect of this situation is the total force required to be applied on the device. If we have a device that has a high number of contact pads, and the force per pad to reach a satisfactory electrical connection, the total mechanical force can damage the device physically. For all the above reasons, designers put a lot of effort in designing soft contact springs. The ideal spring would have a force-deflection curve that starts at zero force for zero deflection and then slopes upward gradually.
Now, if we look at the force-deflection curve of a column, unfortunately it has the opposite shape, or general slope, compared with the desirable/ideal springs. A column stays straight under an increasing force and when the force reaches a critical level, the column buckles and gives way very rapidly. If we study the force-deflection curve of a column, we will see that basically it has a shape and slope contrary to the ideal contact spring. At the beginning, when a force is applied to a column, we see no deflection. Even after increasing the force to a high level, there would still be no deflection. The force-deflection curve is practically a straight vertical line with zero deflection. After the force reaches the buckling level or limit, the column buckles and practically collapses down. The force-deflection curve drops down almost exponentially until it reaches a point where the structure converts to a beam mode under axial loading. The slope of the curve after the initial buckling usually becomes negative. Such a condition is not desirable, especially with any appreciable non-planarity. To reduce this undesirable effect, IBM has opted to provide several steps of buckling, to accommodate more non-planarity than is available with one level. But, in spite of that, the basic force-deflection curve is the same. The only difference is that with the multi-level buckling, we would get a curve that would look like a saw tooth. The force would increase to the buckling limit then collapse at the first buckling, then the force would increase again to a similar buckling level and then collapse at the second buckling and so on.
The contact springs according to the present invention provide the more desirable force-deflection curves as will be describes later. This means that they would not have the tendency of damaging the contact pads of the device under test.
In U.S. Pat. No. 5,385,477, the CK Technology “Contactor with Elastomer Encapsulated Probes” has modified the design of the IBM Buckling Beam slightly. Instead of relying only on the applied force to buckle the beam, CKT has provided means to “nudge” the column to collapse more easily. However, the force-deflection curve is still basically that of a column and it would have a similar vertical spike, for one column segment, and if more than one segment, then the saw tooth shape, as described above. Hence, no drastic improvement.
As mentioned above, the contact springs according to the present invention provide the more desirable force-deflection curves as will be describes later. And again, this means that they would not have the tendency of damaging the contact pads of the device under test.
2. High-Density or Small Pitch.
All the patents listed below in this section have been invented by the present inventor, Gabe Cherian, either solely by him or together with other co-inventors. The reason for listing them is to show that each one of them has one or more features which make them not compatible with the present needs of the industry, from the point of view high density. This is the reason the inventor was motivated to update his old inventions and to address the present market needs.
G. B. Cherian, W. S. Scheingold and S. J. Kandybowski, “Electrical Interconnect Device”, AMP Incorporated, Harrisburg, Pa., U.S. Pat. No. 4,262,986, Apr. 21, 1981. Shows a large footprint compared to the height (excluding the soldertail). Totally opposite of what is being achieved in this invention.
G. B. Cherian, W. S. Scheingold and L. D. Wulf, “Zero Insertion Force Connector”, AMP Incorporated, Harrisburg, Pa., U.S. Pat. No. 4,080,032, Mar. 21, 1978. Only 2 rows of contacts, then we can use the space between the rows or outside the rows.
E. J. Bright, G. B. Cherian and W. S. Scheingold, “Ejection Device for a Electronic Package Connector”, AMP Incorporated, Harrisburg, Pa., U.S. Pat. No. 4,190,310, Feb. 26, 1980. Same. Only 2 rows of contacts, then we can use the space between the rows or outside the rows.
G. B. Cherian, W. S. Scheingold and F. C. Youngfleish, “Active Device Substrate Connector”, AMP Incorporated, Harrisburg, Pa., U.S. Pat. No. 4,341,433, Jul. 27, 1982. Perimeter. Almost like the 2 rows, except at the corners. 2 rows or perimeter: Worry about the center distance only in one direction, but have more room in the direction perpendicular to the former.
G. B. Cherian, W. S. Scheingold, “Connecting Element for Surface To Surface Connectors”, AMP Incorporated, Harrisburg, Pa., U.S. Pat. No. 4,161,346, Jul. 17, 1979. Close but too difficult for real small pitch
G. B. Cherian, W. S. Scheingold and R. D. Zimmeran, “Electrical Interconnection Device”, AMP Incorporated, Harrisburg, Pa., U.S. Pat. No. 4,199,209, Apr. 22, 1980. Close but too difficult for real small pitch
Cherian, Gabe, “Heat Recoverable Connecting Device”, Raychem Corporation, Menlo Park, Calif., U.S. Pat. No. 4,487,465, Dec. 11, 1984 . . . . Close but too difficult for real small pitch.
The IBM “COBRA” has one advantage over the Buckling Beam, but it has a major drawback. It can not be parallel nested close enough to accommodate high-density in both the x- and the y-directions. FIG. 12.A shows the general configuration of the IBM Cobra Spring or Needle. If we place such springs next to each other, but without allowing them to touch, in a direction perpendicular to the plane of the belly, which we would arbitrarily call the y-direction, the center distance between any two adjacent springs would depend on the diameter of the spring. For example, if we want the clearance between any two adjacent springs to be equal to one diameter of the springs, then the center distance would be equal to two diameters. But on the other hand, if we try to parallel nest two or more such springs, in the direction shown in FIG. 12A, which would be in this case the x-direction, the center distance will be considerably larger. The reason is because the C-Shape belly of the IBM Cobra Springs is almost a half circle. In order to prevent the springs from touching, and thus shorting electrically, and have a similar clearance of one diameter between any two adjacent springs, as in the previous case, the springs will have to be placed at a much larger center distance.
In the present invention, we present ways to decrease the center distances between springs, thus accommodating higher densities of contacts pads.
3. Impedance Control
In all the above prior art, the contact springs are bare, i.e. not covered by any insulation or the like. If the spring is long, then its length may affect the quality of the electrical transmission. In the present invention, we present ways to control the impedance of the contact springs and improve their electrical performance.
4. Wipe or Scrub
In all the above prior art, as well as in the Charles Everett “POGO” pins and all similar contact springs, the tips of the springs apply the force at one point of the contact pad. They do not provide “wipe” or “scrub”. If a contact spring provides wipe or scrub, it help in removing the undesirable layers of dirt and/or oxides from the space under the spring and allows the spring to make metal contact under a smaller force, than when there is no wipe or scrub. So, all the prior art mentioned above, by not providing wipe or scrub, require a comparatively high force to make a good electrical connection, compared with a contact spring that does provide the desirable wipe or scrub. The present invention does provide means to have wipe or scrub as will be describes later.
5. TCE Matching
In all the above art and in most sockets or connectors, the springs are located in housings, usually made of plastic. Then the sockets or connectors are mounted on boards, which are usually made of FR4 or glass-filled epoxy material or the like. Then the device under test, for example, a BGA package is placed on top of the socket or connector, and the whole stack would be placed in a thermal cycling oven. The TCE (Thermal Coefficient of Expansion) of these material can vary dramatically. During thermal cycling, these different materials would expand and shrink at different rates. The effect of this can be detrimental, especially for large temperature variations, and for the materials in contact with each other. There would be a “relative motion” between the tips of the springs and the contact pads, both at the socket/board interface, as well as at the socket/package interface.
The present invention covers improvements to this undesirable condition as will be explained later.
6. Elastomer with Dispersed V Springs
U.S. Pat. No. 4,660,165, for the Shin-Etsu “Press-Contact Type Interconnectors” rely on the elastomer to provide some of the contact force. This is not what I am doing here in my invention. Wherever I use an elastomer, it is simply as a means to hold the springs together to facilitate the assembly process. As I have explained in the description, I can use a wax to hold the springs together and after the assembly is done, I can melt the wax away and the socket would work just as well without the wax or the elastomer.